SATELLITES

An object that circles another in space is called its
satellite. Earth has one natural satellite, the Moon, but a swarm of artificial
satellites. The USSR launched the first artificial satellite, Sputnik 1, in
October 1957. Today, thousands of satellites circle Earth in different ORBITS, doing all kinds of jobs, such as relaying phone calls and TV
broadcasts, and monitoring weather.

HOW SATELLITES WORK

Satellites are built of the lightest materials possible to make them
easier to launch. They can carry a wide variety of instruments, such as
cameras, telescopes, radiation sensors, and radio equipment. Panels of solar
cells provide electricity to power the instruments.

SATELLITE COMMUNICATIONS

Earth stations use huge dish antennae to beam radio waves up to
communication satellites. The satellites beam them back down to other Earth
stations in the same or a different country. Many national and most
international telephone calls, emails, and fax messages are now handled by
networks of communications satellites, many of them in geostationary orbit.

ORBITS

Satellites circle in space around Earth in a variety of
paths, or orbits. They are kept in orbit by achieving a balance between their
speed and the force of gravity: speed pushes them outward, while gravity pulls
them inward. The speed needed to stay in orbit at a given height is called
orbital velocity. For orbit at a few hundred kilometres, the orbital velocity
is 28,000 kph (17,500 mph).

Table 19. SATELLITES AND THEIR ORBITS

Low-Earth orbit

Mobile communications, reconnaissance, astronomy

Polar orbit

Weather, navigation

Highly elliptical

Communications at orbit northern latitudes

Geostationary orbit

Communications, weather

Satellites follow different orbits around Earth. In a polar orbit,
they circle over the poles. In a highly elliptical orbit, they swoop close to
Earth at one point but move far away at another. In a geostationary orbit,
36,000 km (23,500 miles) high, they circle every 24 hours and appear fixed in
the sky. A low-Earth orbit is only a few hundred kilometres above Earth.